Monahan is playing as a top-6 C on the Jets and scoring at a top-6 rate this year.
If you see the trade as losing a first-rounder for a secondary piece, it looks like poor value.
If you see it as trading a de facto second-rounder (with commensurately lower likelihood of significant impact) for a now-healthy 2C, who could be a difference-maker in the playoffs, it makes a lot more sense.
In the middle of a losing streak nothing looks good. Over the next 10-20+ games, if he stays healthy, I suspect Monahan will justify his cost, and then some.